Many electrical devices use electrochemical battery cells as power sources. The size and shape of the battery is often limited by the battery compartment of the device. Manufacturers continually try to increase the capabilities and features of electrical devices thereby increasing demands on the batteries used therein. As the shape and size of the battery is often fixed, battery manufacturers must modify cell characteristics to provide increased performance.
Solutions to provide increased performance have included minimizing volume taken up in the cell by the housing, including the seal and vent, as well as reducing the thickness of the separator between the negative electrode (anode) and positive electrode (cathode). Such solutions attempt to maximize the internal volume available for active materials.
It is also desirable to utilize natural pyrite or ion disulfide (FeS2) ore in the positive electrode of an electrochemical cell as an active material as it provides desirable performance characteristics. However, natural iron disulfide ore can contain any of a number of impurities, whether present in the natural product or added intentionally or unintentionally by a mining company or supplier. The type of impurities can vary by source, i.e. samples from different mining regions, or even by lot from the same location. While sampling and lot selection can be utilized to limit impurity types or levels, or a combination thereof, impurities cannot be eliminated entirely. In the past, lithium/iron disulfide batteries were prepared having an iron disulfide average particle size greater than 20 μm, and a median inherent pH value prior to use in an electrode of about 5.0 with pH values ranging from 3.75 to 6.86. Also, lithium/iron disulfide batteries were prepared having an iron disulfide average particle size from 1 to 19 μm, and a median inherent pH value of about 4.75 with pH values ranging from 4.03 to 5.56. Currently, the use of synthetic iron disulfide is cost prohibitive.
The pyrite or iron disulfide particles utilized in electrochemical cell cathodes are typically derived from natural ore which is crushed, heat treated, and dry milled to an average particle size of 20 to 30 microns. The fineness of the grind is limited by the reactivity of the particles with air and moisture. As the particle size is reduced, the surface area thereof is increased and is weathered. Weathering is an oxidation process in which the iron disulfide reacts with moisture and air to form iron sulfates. The weathering process results in an increase in acidity and a reduction in electrochemical activity. Small pyrite particles can generate sufficient heat during oxidation to cause hazardous fires within the processing operation. Prior art iron disulfide particles utilized can have particles sizes which approach the final cathode coating thickness of about 80 microns due to the inconsistencies of the dry milling process.
The dry milling process of iron disulfide is typically performed by a mining company or an intermediate wherein large quantities of material are produced. The processed iron disulfide is shipped and generally stored for extended periods of time before it can be used by the battery industry. Thus, during the storage period, the above-noted oxidation and weathering occur and the material degrades. Moreover, the large iron disulfide particles sizes can impact processes such as calendering, causing substrate distortion, coating to substrate bond disruption, as well as failures from separator damage.
It has been found desirable to improve electrochemical battery cell performance by utilizing smaller average particle size iron disulfide. The average particle size of the iron disulfide is typically reduced via a milling process such as media or jet milling. As a result, the iron disulfide surface area is increased and additional impurity inclusions are exposed and can be released into the electrolyte.
Batteries having iron disulfide positive electrodes and preferably lithium negative electrodes occasionally exhibit defects which result in internal shorting. The defects are believed to be caused by impurities such as metals, for example zinc, which are present in raw material sources such as the iron disulfide. Some of the impurities are soluble in the non-aqueous electrolyte and deposit on the negative electrode as dendrites. The dendrites can grow large enough to form a conductive bridge across the separator and cause an internal short in the electrochemical cell.
For the foregoing reasons, there is a need for electrochemical cells in which internal short circuits or dendrite growth or formation can be reduced or substantially prevented. The prevention of dendrite growth or internal short circuits should not come at the expense of cell performance.